[Intel-gfx] Issue with cec_register_adapter calling request_module() from an async context when called from intel_dp_detect

[Hans Verkuil]

On 17/02/2021 16:11, Sean Young wrote:
> Hi,
> 
> On Wed, Feb 17, 2021 at 04:04:11PM +0100, Hans de Goede wrote:
>> On 2/17/21 3:32 PM, Sean Young wrote:
>>> On Wed, Feb 17, 2021 at 01:41:46PM +0100, Hans Verkuil wrote:
>>>> Hi Hans,
>>>>
>>>> On 17/02/2021 13:24, Hans de Goede wrote:
>>>>> <resend with the linux-media list added to the Cc>
>>>>>
>>>>> Hi Hans,
>>>>>
>>>>> Fedora has a (opt-in) system to automatically collect backtraces from software
>>>>> crashing on users systems.
>>>>>
>>>>> This includes collecting kernel backtraces (including once triggered by
>>>>> WARN macros) while looking a the top 10 of the most reported backtrace during the
>>>>> last 2 weeks report from ABRT: https://retrace.fedoraproject.org/faf/problems/
>>>>>
>>>>> I noticed the following backtrace:
>>>>> https://retrace.fedoraproject.org/faf/problems/8150/
>>>>> which has been reported 170000 times by Fedora users who have opted-in during the
>>>>> last 14 days.
>>>>>
>>>>> The issue here is that cec_register_adapter ends up calling request_module()
>>>>> from an async context, triggering this warn in kernel/kmod.c __request_module():
>>>>>
>>>>>         /*
>>>>>          * We don't allow synchronous module loading from async.  Module
>>>>>          * init may invoke async_synchronize_full() which will end up
>>>>>          * waiting for this task which already is waiting for the module
>>>>>          * loading to complete, leading to a deadlock.
>>>>>          */
>>>>>         WARN_ON_ONCE(wait && current_is_async());
>>>>>
>>>>> The call-path leading to this goes like this:
>>>>>
>>>>>  ? kvasprintf+0x6d/0xa0
>>>>>  ? kobject_set_name_vargs+0x6f/0x90
>>>>>  rc_map_get+0x30/0x60
>>>>
>>>> It's not CEC, it is rc_map_get that calls request_module() for rc-cec.ko.
>>>>
>>>> I've added Sean Young to the CC list.
>>>>
>>>> Sean, is it possible to treat rc-cec as a built-in if MEDIA_CEC_RC is set?
>>>>
>>>> I think this issue is very specific to CEC. I would not expect to see this
>>>> with any other rc keymap.
>>>
>>> So CEC creates an RC device with a keymap (cec keymap, of course) and then
>>> the keymap needs to be loaded. We certainly don't want all keymaps as
>>> builtins, that would be a waste.
>>>
>>> The cec keymap is scanned once to build a map from cec codes to linux
>>> keycodes; making it builtin is not ideal, and makes the build system a
>>> bit messy.
>>>
>>> I don't think we can load the keymap later, user space may start remapping
>>> the keymap from udev.
>>>
>>> Possibly we could create the cec or rc device later but this could be a bit
>>> messy.
>>>
>>> Could CEC specify:
>>>
>>> #if IS_ENABLED(CONFIG_MEDIA_CEC_RC)
>>> MODULE_SOFTDEP("rc-cec")
>>> #endif
>>
>> That would need to be:
>>
>> MODULE_SOFTDEP("pre: rc-cec")
>>
>> I see that the drm_kms_helper and i915 drivers both depend on the cec module already,
>> so yes if that module will request for rc-cec to be loaded before it is loaded
>> (and thus before i915 is loaded) then that should work around this.
>>
>> Assuming the user is using a module-loader which honors the softdep...
>>
>> Also this assumes that rc_map_get is smart enough to not call request_module()
>> if the module is already loaded, is that the case ?
> 
> Yes, see rc_map_get().

I tried this. It works if CONFIG_RC_CORE is set to m, but setting it to
y resulted in the same problem. It looks like MODULE_SOFTDEP only works if rc_main
is a module as well.

It was a good idea, though :-)

Regards,

	Hans